1. Field of the Invention
The present invention relates to an ink composition suitably used for inkjet recording, an inkjet recording method employing the ink composition, and a printed material obtained by employing the ink composition.
2. Description of the Related Art
As image recording methods for forming an image on a recording medium such as paper based on an image data signal, there are an electrophotographic system, sublimation type and melt type thermal transfer systems, an inkjet system, etc. Among these, the inkjet system uses inexpensive equipment and, since an image is formed directly on a recording medium by discharging an ink only on a required image area, the ink can be used efficiently and the running cost is low. Furthermore, there is little noise and it is excellent as an image recording system.
In accordance with the inkjet system, it is possible to print not only on plain paper but also on a non-water absorbing recording medium such as a plastic sheet or a metal sheet, but achieving a higher printing speed and higher image quality are important issues, and the time required for drying and curing droplets after printing greatly influences the sharpness of an image.
As one inkjet system, there is a recording system using an inkjet recording ink that can be cured by exposure to actinic radiation. In accordance with this method, by irradiating with radiation immediately after printing so as to cure ink droplets, it is possible to form a sharp image.
Such a curable ink composition is required to have high pigment dispersibility and stability over time in order to form a high definition image having excellent coloring properties. In order to give the ink composition vivid colors and high coloring power, it is essential to make a pigment very fine. In particular, with regard to an ink used for inkjet recording, since the discharged ink droplets greatly influence the sharpness of an image, the discharged droplets are made small, and it is essential to use particles that are finer than the thickness of a cured ink film formed by the ink. In this way, when pigment particles are made finer in order to obtain high coloring power, it becomes difficult to disperse the fine particles, and aggregates are easily formed. There is also the problem that addition of a dispersant causes the viscosity of the composition to increase. Since formation of pigment aggregates and increase in viscosity of the ink composition both adversely affect the ink dischargeability, it is undesirable to use for inkjet recording an ink composition in which aggregation of pigment, viscosity increase, etc. occur.
Moreover, when an ink composition is used for inkjet recording, since the ink composition is stored in a cartridge, heated during discharge, and cooled when not discharged and when stored, it is exposed to repeated temperature changes from heating and cooling; these temperature changes also adversely affect the pigment dispersibility, the pigment dispersibility deteriorates over time, and there is the problem that pigment aggregation, increase in viscosity, etc. easily occur.
There is therefore a desire for an ink composition that has sufficient fluidity and enables a pigment that has been made very fine to be stably dispersed, and for which a pigment dispersion has excellent stability over time. In order to obtain a stable pigment dispersion various proposals have been made with respect to the dispersant.
In order to improve the affinity for a pigment, an ink composition in which a pigment derivative is used as a dispersant (ref. e.g. JP-A-2003-119414 and JP-A-2004-18656; JP-A denotes a Japanese unexamined patent application publication), an ink composition in which a polymer having a basic group is used as a dispersant for a specific pigment such as a phthalocyanine- or quinacridone-based pigment (ref. e.g. JP-A-2003-321628), and an ink composition comprising a dispersant such as a polyethyleneimine-poly(12-hydroxystearic acid) graft polymer and a specific monomer for dissolving the dispersant but employing no organic solvent (ref. e.g. JP-A-2004-131589) have been proposed. Furthermore, as a dispersant having a secondary amino group in a graft chain, a dispersant comprising a vinyl polymer having a polyethyleneimine in a graft chain (ref. e.g. JP-A-2004-344795) has been proposed.